Gravity sensors and gravity gradiometer instruments in particular, require stabilization for use on moving platforms or vehicles. Stabilization is the means for orienting the instrument in a preferred manner (typically aligned with a North-East-Down detached Earth survey reference frame) and for isolating the instrument from dynamic motion disturbances of its host vehicle. Transmission of motion disturbances from the host vehicle to the gravity instrument result in errors that cannot be completely removed by subsequent processing or by other means.
Host vehicles, airborne vehicles in particular, exhibit translational motion disturbances that are too large to be absorbed/accommodated by a conventional mechanical interface/suspension apparatus between an instrument and the vehicle. There is insufficient sway or rattle space to hold an instrument fixed inertially (e.g. in a desired survey frame) while its host vehicle moves (translates its position) around it. Rotational motion disturbances, however, can be accommodated by a suspension apparatus. Typically, a series of nested gimbals is used. Successive gimbals interconnect with one rotation axis per gimbal via mechanical bearings and races. An outer frame plus three successive inward gimbals provide rotational isolation from host vehicle angular motion disturbances. The outer frame sits atop pedestals and air springs which mount to the host vehicle, providing vibration isolation at frequencies above the natural resonance of this passive isolation arrangement. The outer frame is thus a linear motion stage. The gravity instrument mounts to the innermost gimbal structure. As such, the characteristic dimension of the sensor (proportional to its sensitivity) is much less than that of the overall volume occupied when deployed on a vehicle, thereby resulting in a sub-optimal configuration.
Angular motion disturbances are limited to tens of degrees (vehicle pitch and roll) so electrical continuity (for power and data) is maintained through each of the two outermost gimbals via flex capsules (resembling rotary ribbon cables) and through the innermost gimbal via slip rings due to its unlimited azimuth rotation capability. Care, maintenance, and replacement of the bearings, flex capsules, and slip rings reduces availability.
The use of ball bearings and races between gimbals gives rise to jitter disturbance torques applied to the gimbal structure supporting the gravity instrumentation. Jitter disturbances and jerk-like motions result from bearing stiction, which is the static friction that needs to be overcome to enable relative motion between objects in contact (in this case any pair of gimbals or outer gimbal and frame connected through mechanical bearings). Structural pieces pressing against one another (but not sliding) will require some threshold of force parallel to the contact surface to overcome static cohesion. Stiction is a threshold, not a continuous force. Shock or recoil-like “jitter” disturbances are imparted on the gravity instrumentation when bearing stiction is overcome. The ensuing angular rates imparted to the instrument are broadly referred to as jitter. Although jitter is a measurable effect (using gyroscopes co-mounted with the gravity instruments) the same disturbance also excites additional immeasurable error mechanisms. Alternative systems and methods in a gravity gradiometer instrument that reduce or eliminate such disadvantages are desirable.